Digital bone densitometry devices such as the DPX machines manufactured by LUNAR Corporation of Madison, Wisc. or the QDR machines manufactured by Hologic, Inc. of Waltham, Mass., are used to generate broadly based values of bone character, such as bone mineral content ("BMC") or bone mineral density ("BMD"). Such information about bone character, and in particular, about bone character in the spine is often relied on to diagnose and treat bone depletive disorders such as osteoporosis.
Traditionally, BMC and BMD measurements have been made by scanning the spine of a patient with a radiation source directed along an anterior-posterior ("AP") axis. One problem with AP scans of the spine for BMC and BMD measurement is that the measurement of the diagnostically significant trabecular bone in each vertebra is biased by contribution from the posterior elements of each vertebra. This is because bone from the posterior elements projects into the intervertebral space and overlays much of the vertebral body of an AP view. Thus most of the bone of the posterior elements were invariably included in the AP measurement.
To avoid these problems, manufacturers have resorted to measuring the spine from the lateral position. In the lateral position, it is argued, the region of interest can be easily limited to the vertebral body excluding the posterior elements. Thus, one avoided having the measurement biased by the posterior elements.
Nevertheless, significant problems exist with the lateral view. Because patient thickness is greater in the lateral view, resolution is compromised. For the same resolution as is obtained in the AP view, in the lateral view one must increase the flux of the x-ray beam which leads to an increased dose. If flux was not increased, the ability to define the margins of the vertebral body was no better and in many instances was worse than with the AP view. Further, most of the lateral view of the spine is obstructed by the ribs or the hip. It can be appreciated by those skilled in the art, that such an obstruction presents a similar biasing problem as discussed above with respect to the posterior elements in the AP view.
At best, only two vertebrae, L1 and L2, present an unobstructed lateral view and this is true only for 20 percent of the population. In the small percentage of the population where an unobstructed view is possible, if the vertebrae have a pathology, such as crush fractures, the BMC or BMD measurement of those vertebrae may not be clinically relevant.